1. Field of the Invention
The present invention relates to a vehicle hydraulic pressure control apparatus that executes an anti-skid control (hereinafter referred to as “ABS control”) for preventing an excessive slip of a wheel.
2. Description of the Related Art
Heretofore, a brake hydraulic pressure control apparatus that controls a brake hydraulic pressure in a wheel cylinder (hereinafter referred to as “wheel cylinder hydraulic pressure”) to execute the ABS control has widely been mounted on vehicles. In general, the brake hydraulic pressure control apparatus is provided with a normally-open solenoid valve (pressure-increasing valve) disposed in a hydraulic circuit between a master cylinder generating a brake hydraulic pressure (hereinafter referred to as “master cylinder hydraulic pressure”) according to a brake operation by a driver and the wheel cylinder and a normally-closed solenoid valve (pressure-reducing valve) disposed in a hydraulic circuit between the wheel cylinder and a reservoir, wherein the wheel cylinder hydraulic pressure can be reduced, held and increased by controlling the pressure-increasing valve and the pressure-reducing valve.
The ABS control is, in general, started in response to the establishment of predetermined ABS control start conditions, and is accomplished by performing the pressure-increasing control after the execution of the pressure-reducing control. When the ABS control start conditions are again satisfied during the pressure-increasing control in this-time ABS control, the pressure-increasing control is ended and the next ABS control is continuously started. Specifically, supposing that the period from when the ABS control start conditions are satisfied to when the next ABS control start conditions are satisfied is referred to as one control cycle, the ABS control is, in general, executed continuously plural times over plural-time control cycles.
In the brake hydraulic pressure control apparatus, a normally-open linear solenoid valve that can control a differential pressure (hereinafter referred to as “actual differential pressure”) between the master cylinder hydraulic pressure and the wheel cylinder hydraulic pressure so as to be linear by linearly controlling the energizing current value has recently been adopted as the pressure-increasing valve, based upon a demand for gently increasing the wheel cylinder hydraulic pressure in the pressure-increasing control (see the following Patent Reference 1).
[Patent Reference 1] Japanese Patent Application Laid-Open (kokai) No. 2003-19952
In general, the aforesaid normally-open linear solenoid valve specifies the relationship between the energizing current value (command current) and the differential pressure (command differential pressure) corresponding to suction force by its specification. When the command differential pressure determined according to the energizing current value is greater than the actual differential pressure, the normally-open solenoid valve is closed to break the communication between the master cylinder and the wheel cylinder. On the other hand, when the command differential pressure is smaller than the actual differential pressure, the normally-open linear solenoid valve is opened to establish the communication between the master cylinder and the wheel cylinder. As a result, the brake hydraulic pressure is flown from the master cylinder into the wheel cylinder, thereby being capable of making an adjustment such that the actual differential pressure agrees with the command differential pressure.
Accordingly, in order to execute the pressure-increasing control by using the normally-open solenoid valve, at first, the energizing current value to the normally-open linear solenoid valve (i.e., pressure-increasing valve) is immediately set to the current value (i.e., the energizing current value in order to match the command differential pressure to the actual differential pressure; hereinafter referred to as “current value corresponding to the actual differential pressure”) at the point of starting the pressure-increasing control with the pressure-reducing valve maintained in its closed state. With this state, the energizing current value is gradually decreased, resulting in that the actual differential pressure is gradually decreased. This can gently increase the wheel cylinder hydraulic pressure over the pressure-increasing control.
In other words, in order to gently increase the wheel cylinder hydraulic pressure from the point of starting the pressure-increasing control, it is necessary to correctly obtain the current value corresponding to the actual differential pressure (accordingly, the actual differential pressure at this point) at the point of starting the pressure-increasing control (or before this point). The actual differential pressure can easily be detected by using both a sensor detecting the master cylinder hydraulic pressure and a sensor detecting the wheel cylinder hydraulic pressure. However, using these two sensors is generally difficult to be adopted, since there arises a problem of increasing production cost and a problem of being difficult to secure reliability of the sensors.
From the above, it is necessary to obtain the actual differential pressure (or current value corresponding to the actual differential pressure) at the point of starting the pressure-increasing control without utilizing these sensors. Therefore, the brake hydraulic pressure control apparatus disclosed in the Patent Reference 1 obtains the current value corresponding to the actual differential pressure during the pressure-increasing control in the first-time control cycle (first-time ABS control), and based upon this value, obtains the current value corresponding to the actual differential pressure at the point of starting the pressure-increasing control in the second-time and the following control cycles. This technique will be more specifically explained hereinafter with reference to FIG. 14.
FIG. 14 is a time chart showing one example of a change in wheel speed Vw, (estimated) vehicle body speed Vso, master cylinder hydraulic pressure Pm, wheel cylinder hydraulic pressure Pw and command current value Id (i.e., energizing current value) to the pressure-increasing valve that is the linear solenoid valve, in case where a driver of a vehicle having mounted thereto the brake hydraulic pressure control apparatus executing the above-mentioned technique continuously executes the brake operation from a certain point before time t1 and the ABS control start conditions are satisfied at time t1 and time t4 (i.e., in case where the period from time t1 to time t4 corresponds to the first-time control cycle and the period after time t4 corresponds to the second-time control cycle).
As shown in FIG. 14, this apparatus starts the pressure-reducing control simultaneous with the start of the first-time ABS control at time t1, and when predetermined holding control start conditions are satisfied during this pressure-reducing control, executes the holding control after the pressure-reducing control. Thereafter, since predetermined pressure-increasing control start conditions are satisfied upon having reached time t2, this apparatus sets the command current value Id to a predetermined value at time t2 and gradually decreases the command current value Id in the period from time t2 to time t4, thereby executing the pressure-increasing control.
In this example, the command current value Id is greater than the current value corresponding to the actual differential pressure (i.e., the command differential pressure is greater than the actual differential pressure) during the period from time t2 to time t3, that means the linear solenoid valve is maintained in its closed state. Accordingly, the wheel cylinder hydraulic pressure Pw becomes constant during this period. Upon having reached time t3, the command current value Id agrees with the current value corresponding to the actual differential pressure, so that the linear solenoid valve is opened and the wheel cylinder hydraulic pressure Pw is increasing according to the decrease in the command current value Id during the period from time t3 to time t4. In other words, the command current value Id keeps on agreeing with the current value corresponding to the actual differential pressure during the period from time t3 to time t4. Thus, this apparatus can correctly obtain the current value Idc corresponding to the actual differential pressure at time t4 that is the point of ending the pressure-increasing control.
Subsequently, this apparatus again starts and executes the pressure-reducing control simultaneous with the start of the second-time ABS control at time t4. At this time, this apparatus obtains, by a predetermined technique, a current value ΔIrdc corresponding to the reduced pressure that corresponds to the actual differential pressure increased with the decrease in the wheel cylinder hydraulic pressure during this pressure-reducing control. The current value ΔIrdc corresponding to the reduced pressure can be obtained, for example, as the product of the pressure-reducing control continuation time by a predetermined coefficient.
After executing the holding control after this pressure-reducing control and upon having reached time t5 that is the point when the pressure-increasing control start conditions are satisfied, this apparatus sets the command current value Id to a value ID (ID=Idc+ΔIrdc) obtained by adding the aforesaid “current value ΔIrdc corresponding to the reduced pressure” to the aforesaid “current value Idc corresponding to the actual differential pressure at time t4”. Here, this value ID agrees with the current value corresponding to the actual differential pressure at time t5 (i.e., at the point of starting the pressure-increasing control in the second-time control cycle). Therefore, the command current value Id keeps on correctly agreeing with the current value corresponding to the actual differential value even in the pressure-increasing control executed after time t5, like the previous period from time t3 to time t4, with the result that the current value corresponding to the actual differential value at the point of starting the pressure-increasing control in the third-time and the following control cycles can also be successively and correctly obtained like the aforesaid value ID.
In this manner, this apparatus correctly obtains the current value corresponding to the actual differential pressure at the point of ending the pressure-increasing control (i.e., at the point of ending the first-time pressure-increasing control) by utilizing the fact that the command current value Id keeps on agreeing with the current value corresponding to the actual differential pressure during the period from a certain point during the pressure-increasing control in the first-time control cycle to the point of ending the pressure-increasing control, and based upon this value, this apparatus can correctly obtain the current value corresponding to the actual differential pressure at the point of starting the pressure-increasing control in the second-time and the following control cycles.
However, in case where the actual differential pressure becomes extremely great at the point of starting the pressure-increasing control in the first-time control cycle, there may be the case where the command current value Id does not agree with the current value corresponding to the actual differential pressure during the pressure-increasing control. Specifically, in case where a driver performs a rapid and strong brake operation (i.e., for example, the brake operation wherein brake operation force is rapidly increased over a relatively long period even after the ABS control start conditions are satisfied at time t1 as shown in FIG. 15), the actual differential pressure becomes sufficiently greater than the command differential pressure according to the command current value Id at the point of starting the first-time pressure-increasing control (time t2), whereby the linear solenoid valve that is the pressure-increasing valve is brought into its open state to rapidly decrease the actual differential pressure toward the command differential pressure according to the command current value Id (i.e., to rapidly decrease the current value corresponding to the actual differential pressure toward the command current value Id) after time t2. It should be noted that times t1, t2, t4′ and t5′ in FIG. 15 respectively correspond to times t1, t2, t4 and t5 in FIG. 14.
Thus, the wheel cylinder hydraulic pressure Pw is rapidly increased. At this time, the actual differential pressure becomes extremely great, so that the wheel cylinder hydraulic pressure Pw reaches a value sufficiently great to such a degree that the ABS control start conditions are satisfied, before the current value corresponding to the actual differential pressure reaches the command current value Id as decreasing. As a result, the first-time pressure-increasing control is ended with the state where the current value corresponding to the actual differential pressure Id is maintained to be great at time t4′ that is earlier than time t4 in FIG. 14.
In this case, the value obtained by this apparatus as the current value Idc′ corresponding to the actual differential pressure at time t4′ (i.e., at the point of ending the first-time pressure-increasing control) becomes smaller than the current value corresponding to the actual differential pressure at the actual time t4′, so that the value obtained by this apparatus as the current value ID′ corresponding to the actual differential pressure at time t5′ (i.e., at the point of starting the second-time pressure-increasing control) also becomes smaller than the current value corresponding to the actual differential pressure at the actual time t5′.
Specifically, in this case, the current value corresponding to the actual differential pressure at the point of starting the pressure-increasing control in the second-time control cycle cannot correctly be obtained. The current value corresponding to the actual differential pressure at the point of starting the pressure-increasing control can correctly be obtained from the point of starting the pressure-increasing control in the control cycle next to the point when the command current value Id agrees with the current value corresponding to the actual differential pressure in the second-time or following pressure-increasing control cycle.
In other words, in case where a driver performs a rapid and strong brake operation, the brake hydraulic pressure control apparatus disclosed in the Patent Reference 1 takes much time to correctly obtain the current value corresponding to the actual differential pressure, thereby entailing a problem that the correct and gentle pressure-increasing control utilizing the linear solenoid valve cannot be started at an earlier stage.